1. Field of the Invention
The present invention relates generally to microwave ovens, and more particularly to apparatus and methods for testing said ovens for microwave leakage therefrom.
2. Brief Description of the Prior Art
Manufacturers of microwave ovens have long been concerned with the problem of containing the microwaves used for cooking food in the oven's cooking cavity. The problem exists because the microwaves used for cooking possess certain frequency and wavelength characteristics that allow them to escape from the cavity through the space remaining between the oven's closed door and the edges of the cavity, however perfect the manufacture has been to eliminate this space. Manufacturers have devised many artful methods and means to assist in sealing the door to prevent microwave leakage; nevertheless, no method or means has yet been devised which is one hundred percent effective.
Moreover, the effectiveness of any door seal arrangement is dependant upon a high quality of manufacture. An error or omission during manufacture of the door seal can reduce or eliminate its effectiveness. This could lead to excessive microwave leakage from the oven. A manufacturer, therefore, has an interest in testing each newly manufactured oven for microwave leakage. Additionally, the U.S. government has also in recent years become interested in assuring that each manufactured microwave oven meets minimum standards for microwave leakage and has specified certain requirements for a leakage test. These requirements currently state that a newly manufactured oven allow leakage no more than one (1) milliwatt per square centimeter as measured at any point five (5) centimeters or more from the oven. The government requirements hereinabove specified, and all further references thereto, are specified by the Bureau of Radiological Health, Department of Health, Education and Welfare in its Regulations for Administration and Enforcement of the Radiation Control for Health and Safety Act of 1968, Section 1030.10.
The prior art has generally included two methods of performing such a door leakage test. The first required slowly passing a mobile microwave sensor, usually hand held, in front and along the periphery of the oven door and measuring the power density of leakage at all points; the second required positioning an array of fixed antennae adjacent to the oven door and measuring each antennae to see if at that point the maximum power density had been exceeded.
The first of the above methods has drawbacks which include: (1) increased time to perform the test over the time required for a fixed antennae system, (2) fatigue on moving parts and wires if the sensor is mechanically moved, and (3) the inevitable inaccuracies resulting from human fallibility if the sensor is hand held.
The second of the above methods has drawbacks related to the array positioning vis-a-vis the oven and the spacing between antennae. The positioning and spacing of the array and antennae is chosen somewhat arbitrarily and provides little or no useful information concerning leakage radiation between antennae. Specifically, prior art antenna arrays provide no means of detecting narrow beams of microwaves emitting from the periphery of the door, let alone determining their maximum power density. These beams, especially very narrow ones, can and do pass between adjacent antennae unnoticed and undetected.
Further, spacing such an array away from the oven further than five centimeters, the test distance required by government standards, in order to detect narrow beams does not provide a satisfactory test method because there exists no means for determining from this "far field" information accurate information concerning the shape and power density characteristics of the "near field" unless both amplitude and relative phase are measured and recorded at each sample point. No practicable instrumentation has heretofore been developed to measure simultaneously both amplitude and phase of the external field of a microwave oven, or, further to compute the field at five centimeters (for example) from the data taken at ten centimeters.
Thus the government has decertified all prior art testing methods using such arbitrarily positioned fixed antenna arrays.